At the present time, merchandising of an optical disc having a recording density of approximately 100 GB is possible for consumer use as well owing to Blu-ray Disc™ standard using a blue-violet semiconductor laser. Hereafter, implementation of a large capacity exceeding 500 GB is desired in the optical disc as well. For implementing such an ultrahigh density in the optical disc, however, a high density technique according to a new system different from the conventional high density technique using a shorter wavelength and a higher NA of the object lens is necessary.
In the midst of studies concerning a storage technique of next generation, the hologram recording technique of recording digital information by utilizing the holography attracts attention. As for the hologram recording technique, there is, for example, JPA-2004-272268 (Patent Literature 1). JP-A-2004-272268 describes the so-called angular multiplexing recording system, in which different page data is displayed on a spatial light modulator while an angle of incidence of a reference beam onto optical information storage medium is changed and multiplexing recording is conducted. In addition, JP-A-2004-272268 describes a technique of shortening the spacing between adjacent holograms by focusing a signal beam with a lens and disposing an aperture (spatial filter) in its beam waist.
Furthermore, as for the hologram recording technique, there is, for example, WO 2004/102542 (Patent Literature 2). In an example using a shift multiplexing system described in WO 2004/102542, a beam from inner pixels is used as a signal beam and a beam from outer strip shaped pixels is used as a reference beam in one spatial light modulator. Both beams are focused onto optical information storage medium by using the same lens. The signal beam and the reference beam are caused to interfere with each other in the vicinity of a focal plane of the lens, and holograms are recorded.
Also, as technology of optimizing a reference beam angle at the time of reproduction, there is, for example, JP-A-2011-227967 (Patent Literature 3). In JPA-2011-227967, there is description “a photodetector detects the diffracted beam diffracted by radiating the adjusting beam having the frequency and phase distributions partially the same as those of the signal beam upon recording to the optical information storage medium, and an angel at which a reference beam is incident on the optical information storage medium is controlled based on detected information.”
Also, as technology of disposing light-shielding means before a photodetector at the time of reproduction, there is, for example, JP-A-2011-146098 (Patent Literature 4). In JP-A-2011-146098, there is description “According to a hologram recording/reproduction device of the present invention, at the time of data reproduction, a first region in which one of an information recovered beam and a reference recovered beam is incident on an optical member for light-shielding and a second region in which the other one is incident are set by optical member control means for light-shielding. Also, optical characteristics of pixels arranged in the first region are controlled such that a beam incident on the first region is incident on a light-receiving part, and optical characteristics of pixels arranged in the second region are controlled such that a beam incident on the second region is not incident on the light-receiving part. Since the optical characteristics of pixels of an optical member for light-shielding are controlled as describe above, the beam incident on the first region (one beam of the information recovered beam and the reference recovered beam) is incident on the light-receiving part and the beam incident on the second region (the other beam of the information recovered beam and the reference recovered beam) is not incident on the light-receiving part.”